Coaxial light source with series impedance within the envelope



Nov. 18, 1969 E. L. GARWIN COAXIAL LIGHT SOURCE WITH SERIES IMPEDANCE WITH THE ENVELOPE Filed NOV. 22, 1967 57 INVENTOR.

EDWARD L. GARWIN ATTORNEY.

United States Patent O 3,479,555 COAXIAL LIGHT SOURCE WITH SERIES IMPED- ANCE WITHIN THE ENVELOPE Edward L. Garwin, Los Altos Hills, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Nov.'22, 1967, Ser. No. 685,179 Int. Cl. H01j 19/80, 7/46 U.S. Cl. 31539 10 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The present invention relates generally to pulsed spark gap lamps, and more particularly, it pertains to a device for generating high intensity nanosecond light pulses.

The invention disclosed herein was made under, or in, the course of Contract No. AT(043)-400 with the United States Atomic Energy Commission.

Nanosecond light pulse sources are used in photography of fast events, testing and calibrating light-sensitive instruments, study of the kinetics of very fast luminescent systems and the like. More specifically, a lamp giving repetitious light pulses in the subnanosecond range and at high intensities is of advantage, e.g., to meet the stringent requirements of nanosecond fluorimeters for re solving and measuring the emission kinetics of chromophores, such as for exciting tryptophan and tryosine residues in proteins which have excited state lifetimes greater than one nanosecond. The details of such a fluorimeter are described by Hundley, Coburn, Garwin and Stryer, A Nanosecond Fluorimeter in The Review of Scientific Instruments, vol. 38, No. 4, pp. 488-492, April 1967.

SUMMARY OF THE INVENTION In brief, the pulsed light source lamp of the invention is provided as an elongated, transparent, pressurized gas tube having electrodes in one end, and with a series resistor axially aligned in the other end within an exterior coaxial shield, providing a superior arrangement of the resistive-capacitive components necessary to provide relaxation oscillatory discharges across the electrodes on application of a high driving electrical potential. A very high-intensity electrical discharge lamp is thereby provided, which delivers shaped light pulses of subnanosecond duration and at a high repetition rate beyond the limits of prior art devices. These results are obtained, in part, by minimizing the stray capacitance to ground of the high-voltage electrode. The charging resistance is mounted within the lamp envelope so that the high-voltage electrode lead-in is made as short as possible to reduce stray capacitance due to the length of the lead. The internal mount for the charging resistance elements and the gaseous atmosphere constitute a dielectric that results in a low stray capacitance acting as an exceedingly fast discharge capacitive element in the relaxation oscillation circuit. Mounting of the charging resistances in a pressurized dry gas inhibits undesired arcing-over during high-voltage operation, and the coaxial arrangement provides a very low impedance path, further improving the rise-time discharge characteristics of the device. Arcingover is further inhibited by segmenting the charging resistance as a series of resistors which present a large surface area to provide effective cooling. The use of a series of resistors also reduces the stray capacitance of the high-voltage electrode to a value less than that obtained with a single resistor. The lamp of the present invention is also provided with means for external adjustment of the electrode spacing to obtain optimum intensity of light pulses during operation of the lamp. By using oxygen or other gas pressurized to suitable levels, and by utilizing tubes of appropriate bore diameter, clouding of the envelope is minimized. The lamp is further provided with removable seals for the electrode leads, and with a radio noise shield that is structurally incorporated with the seals. Manufacture of the lamp is simple, and there is no need to resort to complex manufacturing procedures. The seals may be opened, and the lamp completely dismantled to replace any of the parts of the'lamp or to clean any parts of the lamp, especially the electrodes and the light-emitting area of the envelope. The ease of dismantling the lamp also permits flushing of the gas from type of gas.

It is an object of the invention to minimize the stray capacitance and otherwise improve the discharge rate characteristics of the high-voltage electrode in an electrical discharge lamp, to provide for subnanosecond pulse duration operation.

Another object is to provide a device for producing high intensity, subnanosecond light pulses having a shape suitable for use in measuring the emission kinetics of chromophores at a high pulse repetition rate.

Another object is to inhibit high voltage arc-over of the charging resistance of a subnanosecond electrical discharge lamp operating in the relaxation mode.

Another object is to provide a subnanosecond spark gap lamp having adjustable electrodes and removable seals that permit dismantling of the lamp.

Other objects and advantageous features of the invention will be apparent in a description of a specific embodiment thereof, given by way of example only, to enable one skilled in the art to readily practice the invention, and described hereinafter with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The figure is a cross sectional view of a subnanosecond pulse discharge lamp according to the invention.

DESCRIPTION OF AN EMBODIMENT Referring to the figure, there is shown a subnanosecond electrical puuse discharge gas lamp 10 in cross section. The lamp includes a ground electrode 12 and a high-voltage electrode 13, of refractory metal, and spaced apart to form a gap across which subnanosecond electrical discharges, productive of subnanosecond light pulses, are generated. The electrodes 12 and 13 are coaxially mounted within an envelope 15 which may conveniently be a quartz tube for transmission of a broad band of light. The envelope 15 is fitted coaxially within lengths of electrically conductive tubing which may conveniently be copper tubing. The tubing is comprised of an upper section 17 and a lower section 18 that are joined in the area of the sprak gap by a T-joint 20. The transverse leg of the joint 20 is arranged to be directly opposite the discharge gap. The sections 17, 18 and joint 20 thereby act as a light shield to transmit a light beam of a selected diameter in a desired direction. The tubing sections 17, 18 and 20 also act as a light shield so that the light beams are selectively transmitted only through the transverse leg of joint 20. The sections 17 and 18 and joint 20 further act as a noise shield against RF electromagnetic radiation at the discharge gap during generation of a light pulse. For best shielding, a copper mesh screen of large percentage open area should be suitably fastened across the open end of the transverse leg of joint 20. The sections 17, 18 and joint 20 also act as a structural support for the envelope 15 and for the gastight seals at the upper and lower ends of the envelope. The sections and joint also constitute the ground plate of the capacitance to ground of the electrode 13.

The upper end of the envelope 15 is sealed by means of a plastic plug 22, with O-rings 24 circumferentially disposed at the lower end of the plug for forming a gastight seal with the envelope 15. The plug 22 is enclosed by a length of tubing 26, a washer 28, and a cap 30. The tubing 26 and washer 28 may conveniently be made of copper and soldered together. The washer is soldered to the tubing 17 to form an integral supporting structure. The cap 30 may be fastened to the tubing 26 by means of a screw fastener to permit removal of the plug 22 from the envelope 15. A central opening is provided in the cap 30 to permit a reduced diameter cylindrical portion 32 of the plug 22 to extend therethrough. The cylindrical portion 32 is formed with a partially threaded axial hole for receiving a female connector 34. The unthreaded portion of the hole extends into the main body of the plug 22 and further extends at a reduced diameter through the plug 22. The upper end of a high-voltage lead 36 is soldered in the lower part of the connector 34. The connector 34 and lead 36 are sealed in the plug 22 by means of a small rubber ring 38. The lead 36 extends through the plug 22; and the lower end of the lead is connected to a series of resistors 40, mounted within the envelope 15, which are connected to the high-voltage electrode 13 by means of a very short connector 42. The electrode 13 and resistors 40 are coaxially mounted within the envelope 15 by means of a plastic spider guide 43 having a high-voltage breakdown strength.

The lower end of the envelope 15 is sealed by means of a small plug 44 with O-rings 46, disposed circumferentially thereabout to form a gas-tight seal between the envelope and plug. The cylindrical plug 44 is provided with a threaded axial hole for receiving a steel gland nut 48. The ground electrode 12 extends through an axial hole in the nut 48 and a reduced diameter axial hole in the plug 44 into the interior of the envelope 15. A small rubber O-ring 50 provides a gas-tight seal with the nut 48, electrode 12 and plug 44. The nut 48 provides a support and bearing surface for the electrode 12 to permit axial adjustment of the electrode. Adjustment of the electrode 12 is provided for by a supporting base 51 that is integrally secured by suitable means to the tubing 18. The base 51 is provided with a partially threaded axial hole for receiving a threaded tubular fitting 53. The fitting 53 is provided with an insert 55 of electrical insulation having an axial hole in which an adjusting pin 57 is force-fitted so that the fitting 53, insert 55, and pin 57 act as an integral unit. The upper end of the pin 57 is provided with a socket 58 for receiving the lower end of the ground electrode 12. The socket 58 is provided with a retaining tab that extends into a mating groove in the electrode 12 for sliding engagement therewith. Thus, by turning the threaded fitting 53 in the base 51, the pin 57 may be turned with respect to the electrode, yet it will move the electrode 12 axially for adjustment of the spark gap.

The envelope 15 is filled with a pressurized, gaseous atmosphere, for example, oxygen, nitrogen, hydrogen, argon, helium, sulfur hexafluoride, etc., which gives an ultraviolet light pulse. The gas may be supplied through a small passage 60 in the plug 22 which connects the inner space of the envelope 15 with a pressurized gas source. The connection to the gas source is by means of a fitting 62 extending through an opening in the tubing 26 into a threaded hole in the plug 22 which connects with the passage 60. Oxygen is a preferred gas, since overall performance as respects high light output, reproducible amplitude, short pulse duration, long operating life and the like, is best with this gas. Sulfur hexafluoride also gave a very high light output, but some difiiculties, such as amplitude variation and longer pulse durations than with oxygen, were noted.

In operation, the pin 57 is connected to the system ground, while the female connector 34 receives a high voltage pin connected to a high-voltage DC source (not shown). The high-voltage source supplies a charging current through the resistors 40 to the high-voltage electrode 13, charging the capacitance existing principally between ground shield section 17 and the electrode 13 to a voltage that will cause a discharge between the electrodes 12 and 13. Some minor contribution to the capacitance may be provided by the resistor nearest the electrode and shield section 17. Successive charging and discharging of the electrodes is thereby carried out in the relaxation oscillator mode. Spark discharges are produced at the gap at a repetition rate determined by the particular values of resistance and capacitance and voltage applied thereto, which rate can be very high, since the capacity to ground of the electrode 13 is minimized according to the principles of the invention, and a very low inductance path is provided by the coaxial arrangement. The effective RC values of the relaxation mode oscillator is thereby made to have a very short time constant. Moreover, ringing which would produce spurious pulse output and amplitude variation is minimized by terminating the lower electrode in a characteristic impedance coaxial line.

Furthermore, it will be observed that the ground electrode 12 may be removed from the lamp for replacement or cleaning by unscrewing the fitting 53. The high-voltage electrode 13 also may be removed for cleaning or replacement, along with the resistors 40 and the guide 43, by removing the cap 30 and the fitting 62, to permit removal of the plug 22 with the mentioned parts. With the lamp 10 dismantled, the inner surface of the envelope 15 may be easily cleaned of any deposits resulting from sustained operation of the spark gap.

A subnanosecond electrical discharge lamp exemplifying the invention was constructed. The envelope 15 was made of a quartz tube (Suprasil) having an inside diameter of 11 mm. and an outside diameter of 13 mm. The electrodes 12 and 13 were made of 0.0625-inch diameter 2% thoriated tungsten, spaced 0.030 inch. Best results were obtained with the upper electrode ground to a conical point of 45 included angle, while the lower electrode was rounded to a radius of curvature of about 0.1 inch, and with the pointed electrode at positive potential. The charging resistance 40 was comprised of six 22-megohm quarter-watt resistors. The lamp was charged by a 0-30 kilovolt, variable DC source. The lamp was supplied with pure oxygen at a gauge pressure of p.s.i. At 12 kv. a 6- ampere subnanosecond pulse of current discharged across the electrodes; however, the charging current, being steady, flows at a much lower rate tolerated by the resistors.

In operation, light pulses produced by the lamp were found to have an intensity of 6x10 photons per pulse in the wavelength interval from 230 to 470 III/1.. The pulse repetition rate was 2.1 kHz., using oxygen 80 p.s.i.g. and a DC voltage of 14 kv. The spectrum of the light pulses was approximately flat between 200 and 600 Inn, and extended substantially beyond these limits. The rise and fall times of the light pulses, as measured with a planar photodiode, were 0.7 nanosecond and 1.1 nanoseconds, respectively. The accompanying current pulse discharging across the electrodes had rise and fall times shorter than 0.43 nanosecond. When operating at high pressures, e.g., 80 p.s.i.g. or more, hundreds of hours of use without clouding was achieved. At subatmospheric pressures, clouding from Heliarc-welded thoriated tungsten electrodes was very rapid.

The high output of light at 280 m was found especial- 1y useful for excitation of tryptophan and tryosine residues in proteins.

While embodiment of the invention has been shown and described, further embodiments or combinations of those described herein will be apparent to those skilled in the art without departing from the spirit of the invention or from the scope of the appended claims.

I claim:

1. An electrical discharge lamp for generating intense subnanosecond light pulses, comprising:

a transparent envelope with a high-voltage electrode and a ground electrode mounted axially therein, said electrodes being spaced apart to constitute an electrical discharge gap;

means for sealing said envelope;

a high-voltage charging lead passing through said sealing means;

a charging resistance mounted axially within said envelope, said resistance having first and second ends, said first end being connected to said high voltage electrode, and said second end being connected within said envelope to said high-voltage charging lead; and

tubular conductor shield means arranged coaxially with respect to at least said high voltage electrode providing a low capacitance therewith.

2. An electrical discharge lamp according to claim 1,

further including:

a guide support for maintaining said high-voltage electrode and said charging resistance in aligned coaxial position within said envelope, said support having a high-voltage breakdown strength.

3. An electrical discharge lamp according to claim 1, wherein said charging resistance is elongated to maintain a low-voltage gradient between said first and second ends.

4. An electrical discharge lamp according to claim 3, wherein said charging resistance is comprised of a plurality of axially spaced, serially-connected resistors.

5. An electrical discharge lamp according to claim 3, further including means for filling said envelope with a dry pressurized gas.

6..An electrical discharge lamp according to claim 5, wherein said gas is a material selected from the group consisting of oxygen and sulfur hexafluoride.

7. An electrical discharge lamp according to claim 6, wherein said gas is oxygen at a pressure of at least 80 p.s.1.g.

8. A lamp according to claim 1, wherein said envelope is of an elongated tubular configuration, and said tubular conductor shield means is an electrically conductive coaxial tubular member surrounding said envelope, said member also acting as an electromagnetic radiation shield and as a light shield, said member having an opening for directing light pulses from said gap to a selected location outside said envelope.

9. A lamp according to claim 8, wherein said tubular member is a structural support member for said sealing means, and further including a base member on which said tubular member is mounted; and

wherein said sealing means comprises a plug secured between said base member and said tubular member and said envelope, an O-ring circumferentially disposed on said plug, and a gland nut and sealing washer embedded in said plug;

said ground electrode extending through said gland nut, washer and plug into said envelope to form one pole of said spark gap;

further including a threaded fitting in said base member; and

an adjusting means coaxially mounted in said fitting, and having a socket for coaxial engagement with said ground electrode, said adjusting means being mechanically integral with said fitting for axially moving said ground electrode through said sealing means upon rotation of said fitting in said base member for adjustment of said spark gap.

10. A lamp according to claim 9, wherein said sealing means further comprises:

a second plug in said envelope for sealing said highvoltage lead, O-rings circumferentially disposed around said plug between said plug and said envel-ope;

a short length of tubing of a diameter larger than said tubular member and integrally secured thereto and enclosing a portion of said plug;

a female connector embedded in said plug, said highvoltage lead being electrically connected to said connector and sealed in said plug, and passing there through to said second end of said charging resistance; and

a cap removably secured to said short length of tubing for maintaining said plug in said envelope.

References Cited UNITED STATES PATENTS 2,937,299 5/1960 Nolan -1-.. 31559 X 2,977,492 3/1961 Hoekstra 313184 X 3,336,500 8/1967 Sarbach et al. 31559 JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner U.S. Cl. X.R. 

